Liquid crystal display of ocb or va mode

ABSTRACT

A liquid crystal display of OCB mode or VA mode comprises a backlight unit, a backlight-side polarizing plate, a liquid crystal cell of OCB mode or VA mode, and a viewer-side polarizing plate in order. The viewer-side polarizing plate comprises a first transparent protective film, a polarizing membrane, a second transparent protective film and a light-diffusing layer in order. In the liquid crystal display of OCB mode, an optically anisotropic layer is formed from liquid crystal compound on the first transparent protective film. The first transparent protective film is a cellulose acetate film having a Re retardation value of 20 to 70 nm and a Rth retardation value of 100 to 500 nm. The light-diffusing layer comprises transparent resin and transparent fine particles dispersed therein. The transparent resin and the transparent fine particles have refractive indices that are different from each other.

FIELD OF INVENTION

The present invention relates to a liquid crystal display of OCB or VAmode using a polarizing plate.

BACKGROUND OF INVENTION

A liquid crystal display comprises a polarizing plate and a liquidcrystal cell. Various display modes of the liquid crystal cell have beenproposed, for example TN mode and STN mode, which are now popularlyused. In a liquid crystal display of TN or STN mode, an opticalcompensatory sheet is generally provided between the polarizing plateand the liquid crystal cell to improve the viewing angle and qualitiesof the displayed image.

U.S. Pat. Nos. 4,583,825 and 5,410,422 disclose a liquid crystal displaycomprising a liquid crystal cell of OCB (Optically Compensatory Bend)mode, in which alignment of rod-like liquid crystal molecules in upperpart is essentially reversal (symmetrical) to the alignment of moleculesin lower part. Since the liquid crystal molecules are symmetricallyaligned in the upper and lower parts, the liquid crystal cell has aself-optical compensatory function. Therefore, this mode is referred toas OCB (optically compensatory bend) mode. In addition to theself-optical compensatory function, the liquid crystal display of OCBmode has another advantage of a rapid response.

Japanese Patent Provisional Publication No. 2(1990)-176625 discloses aliquid crystal display comprising a liquid crystal cell of VA (VerticalAlignment) mode, in which rod-like liquid crystal molecules areessentially vertically aligned while voltage is not applied (whilenormally black) and the molecules are essentially horizontally alignedwhile voltage is applied. The liquid crystal display of VA mode hasadvantages of giving an image with high contrast, responding rapidly andpreventing the image from undesired coloring.

As described above, the liquid crystal displays of OCB and VA modes haveexcellent characters in displaying images, as compared with popularlyused displays of TN and STN modes. Nevertheless, in consideration ofimages given by a CRT display, it is still necessary to improve thedisplays of OCB and VA modes.

In the displays of OCB and VA modes, as well as in those of TN and STNmodes, optical compensatory sheets may be used to improve displayedimages.

Japanese Patent Provisional Publication No. 09(1997)-197397 (U.S. Pat.No. 5,805,253) and PCT Publication No. 96/37804 (European PatentPublication No. 0,783,128) disclose a liquid crystal display of OCB modeequipped with an optical compensatory sheet comprising discotic liquidcrystal compound. The optical compensatory sheet remarkably improves theviewing angle of the liquid crystal display of OCB mode.

Japanese Patent Provisional Publication No. 2001-027706 discloses aliquid crystal display of VA mode equipped with an optical compensatorysheet comprising discotic liquid crystal compound.

However, though improving image qualities, the optical compensatorysheet provided between the liquid crystal cell and the polarizing plategenerally thickens the display. Further, such display is easilydistorted by heat to leak light since the compensatory sheet increaseselements constituting the display.

Japanese Patent Provisional Publication No. 11(1999) 316378 discloses anelliptically polarizing plate for OCB mode (or for horizontal alignmentmode) in which an optical compensatory sheet and a polarizing plate areunified. In detail, the optical compensatory sheet comprises atransparent support and a thereon-provided optically anisotropic layerformed from discotic liquid crystal compound, and works also as atransparent protective film of the polarizing plate. The thus-unifiedpolarizing plate improves displayed images without thickening thedisplay.

However, it is difficult for even the unified polarizing plate to fullyprevent large displays of 17-inches or more (which have recently beenproduced and used) from thermal distortion and accordingly from leakinglight.

SUMMARY OF INVENTION

The present inventor has studied and found that the light leakage of aliquid crystal display of OCB or VA mode is caused according to thefollowing two mechanisms.

The first cause is distortion induced by change of external temperatureand humidity. When the temperature or humidity changes, a polymer filmused in the optical compensatory sheet expands or shrinks. The expansionor shrinkage is limited since the film is fixed, and accordingly thecompensatory sheet is distorted to change its optical characters. Theother is distortion induced by internal thermal distribution in theoptical compensatory sheet. Heat generated by an internal or neighboringheat source such as a backlight unit gives thermal distribution, whichthermally distorts the sheet to change its optical characters.

Accordingly, in order to prevent the light leakage, it is desired tomake the optical compensatory sheet less change its optical characters.

In the unified polarizing plate or optical compensatory sheet, acellulose acetate film is usually used. However, it has been found thata polymer having hydroxyl group such as cellulose acetate is greatlyaffected with environmental conditions to change its optical characters.

The inventor has further studied and finally found that the unifiedpolarizing plate, in which an optical compensatory sheet and apolarizing plate is unified, can be prevented from changing its opticalcharacter and accordingly from leaking light by thinning two celluloseacetate films used as protective films provided on both sides of thepolarizing membrane.

An object of the present invention is to improve qualities of imagesgiven by a liquid crystal display of OCB mode, by means of a polarizingplate comprising in order an optically anisotropic layer formed fromliquid crystal compound, a first transparent protective film ofcellulose acetate film, a polarizing membrane, a second transparentprotective film, and a light-diffusing layer.

Another object of the invention is to improve qualities of images givenby a liquid crystal display of VA mode, by means of a polarizing platecomprising in order a first transparent protective film of celluloseacetate film, a polarizing membrane, a second transparent protectivefilm, and a light-diffusing layer.

A further object of the invention is to improve image qualities(particularly, to enlarge the viewing angle) of a liquid crystal displaydesigned to give a wide viewing angle, by means of an opticallyanisotropic layer made of optically anisotropic cellulose acetate orliquid crystal compound.

A furthermore object of the invention is to provide a liquid crystaldisplay give a wide viewing angle and to prevent the displayed imagesfrom lowering contrast, from inverting tone and from changing hueaccording to the viewing angle, without thickening the display, by meansof a polarizing plate having improved durability.

The objects of the invention are achieved by the liquid crystal displays(1) to (33) described below.

(1) A liquid crystal display of OCB mode which comprises a backlightunit, a backlight-side polarizing plate, a liquid crystal cell of OCBmode and a viewer-side polarizing plate in order, wherein theviewer-side polarizing plate comprises an optically anisotropic layerformed from liquid crystal compound, a first transparent protective filmof cellulose acetate film, a polarizing membrane, a second transparentprotective film and a light-diffusing layer in order, said viewer-sidepolarizing plate being so placed that the optically anisotropic layerformed from liquid crystal compound is arranged on a side of the liquidcrystal cell, wherein the first transparent protective film is acellulose acetate film having a Re retardation value of 20 to 70 nm anda Rth retardation value of 100 to 500 nm, and wherein thelight-diffusing layer comprises transparent resin and transparent fineparticles dispersed therein, said transparent resin and said transparentfine particles having refractive indices that are different from eachother.

(2) The liquid crystal display of (1), wherein the first transparentprotective is a cellulose acetate film having a thickness of 10 to 70μm.

(3) The liquid crystal display of (1), wherein the first transparentprotective film is a cellulose acetate film comprising cellulose acetatehaving an acetic acid content of 59.0 to 61.5%.

(4) The liquid crystal display of (1), wherein the first transparentprotective film is a cellulose acetate film comprising 100 weight partsof cellulose acetate and 0.01 to 20 weight parts of an aromatic compoundhaving at least two aromatic rings.

(5) The liquid crystal display of (1), wherein the second transparentprotective film is a cellulose acetate film having a thickness of 10 to70 μm.

(6) The liquid crystal display of (1), wherein the second transparentprotective film is a cellulose acetate film comprising cellulose acetatehaving an acetic acid content of 59.0 to 61.5%.

(7) The liquid crystal display of (1), wherein the second transparentprotective film has, on a side of the light-diffusing layer, a surfaceon which average surface roughness measured at a cut-off value of 0.8 mmper 100 mm length is 0.2 μm or less.

(8) The liquid crystal display of (1), wherein the liquid crystalcompound is a discotic liquid crystal compound.

(9) The liquid crystal display of (1), wherein the difference betweenthe refractive index of the transparent resin and the refractive indexof the transparent fine particles is in the range of 0.02 to 0.15.

(10) The liquid crystal display of (1), wherein the transparent fineparticles have a size distribution having at least two peaks.

(11) The liquid crystal display of (10), wherein one peak is in therange of 0.5 to 2.0 μm and another peak is in the range of 2.0 to 5.0μm.

(12) The liquid crystal display of (1), wherein the light-diffusinglayer has a haze of 40% or more.

(13) The liquid crystal display of (1), wherein a low-refractive indexlayer having a refractive index of 1.35 to 1.45 is provided on thelight-diffusing layer.

(14) The liquid crystal display of (13), wherein the low-refractiveindex layer is formed by cross-linking and hardening a composition withheat or ionizing radiation, said composition comprising afluorine-containing compound and inorganic fine particles.

(15) The liquid crystal display of (13), wherein a surface of thelow-refractive index layer shows an integrating spherical averagereflection of 2.3% or less in a wavelength range of 450 to 650 nm.

(16) The liquid crystal display of (1), wherein the liquid crystal cellof OCB mode has a color filter, and a distance between the color filterand the light-diffusing layer of the viewer-side polarizing plate is 0.6mm or less.

(17) The liquid crystal display of (1), wherein the liquid crystal cellof OCB mode comprises a backlight-side substrate, a liquid crystal layerand a viewer-side substrate in order, wherein a color filter is placedbetween the liquid crystal layer and the viewer-side substrate, andwherein a total thickness of the viewer-side substrate, the opticallyanisotropic layer of the viewer-side polarizing plate, the firsttransparent protective film of the viewer-side polarizing plate, thepolarizing membrane of the viewer-side polarizing plate and the secondtransparent protective film of the viewer-side polarizing plate is 0.6mm or less.

(18) A liquid crystal display of VA mode which comprises a backlightunit, a backlight-side polarizing plate, a liquid crystal cell of VAmode and a viewer-side polarizing plate in order, wherein theviewer-side polarizing plate comprises a first transparent protectivefilm of cellulose acetate film, a polarizing membrane, a secondtransparent protective film and a light-diffusing layer in order, saidviewer-side polarizing plate being so placed that the first transparentprotective film is arranged on a side of the liquid crystal cell,wherein the first transparent protective film is a cellulose acetatefilm having a Re retardation value of 20 to 70 nm and a Rth retardationvalue of 100 to 500 nm, and wherein the light-diffusing layer comprisestransparent resin and transparent fine particles therein-dispersed, saidtransparent resin and said transparent fine particles having refractiveindices that are different from each other.

(19) The liquid crystal display of (18), wherein the first transparentprotective film is a cellulose acetate film having a thickness of 10 to70 μm.

(20) The liquid crystal display of (18), wherein the first transparentprotective film is a cellulose acetate film comprising cellulose acetatehaving an acetic acid content of 59.0 to 61.5%.

(21) The liquid crystal display of (18), wherein the first transparentprotective film is a cellulose acetate film comprising 100 weight partsof cellulose acetate and 0.01 to 20 weight parts of an aromatic compoundhaving at least two aromatic rings.

(22) The liquid crystal display of (18), wherein the second transparentprotective film is a cellulose acetate film having a thickness of 10 to70 μm.

(23) The liquid crystal display of (18), wherein the second transparentprotective film is a cellulose acetate film comprising cellulose acetatehaving an acetic acid content of 59.0 to 61.5%.

(24) The liquid crystal display of (18), wherein the second transparentprotective film has, on a side of the light-diffusing layer, a surfaceon which average surface roughness measured at a cut-off value of 0.8 mmper 100 mm length is 0.2 μm or less.

(25) The liquid crystal display of (18), wherein the difference betweenthe refractive index of the transparent resin and the refractive indexof the transparent fine particles is in the range of 0.02 to 0.15.

(26) The liquid crystal display of (18), wherein the transparent fineparticles have a size distribution having at least two peaks.

(27) The liquid crystal display of (26), wherein one peak is in therange of 0.5 to 2.0 μm and another peak is in the range of 2.0 to 5.0μm.

(28) The liquid crystal display of (18), wherein the light-diffusinglayer has a haze of 40% or more.

(29) The liquid crystal display of (18), wherein a low-refractive indexlayer having a refractive index of 1.35 to 1.45 is provided on thelight-diffusing layer.

(30) The liquid crystal display of (29), wherein the low-refractiveindex layer is formed by cross-linking and hardening a composition withheat or ionizing radiation, said composition comprising afluorine-containing compound and inorganic fine particles.

(31) The liquid crystal display of (29), wherein a surface of thelow-refractive index layer shows an integrating spherical averagereflection of 2.3% or less in a wavelength range of 450 to 650 nm.

(32) The liquid crystal display of (18), wherein the liquid crystal cellof VA mode has a color filter, and a distance between the color filterand the light-diffusing layer of the viewer-side polarizing plate is 0.6mm or less.

(33) The liquid crystal display of (18), wherein the liquid crystal cellof VA mode comprises a backlight-side substrate, a liquid crystal layerand a viewer-side substrate in order, wherein a color filter is placedbetween a liquid crystal layer and a viewer-side substrate, and a totalthickness of the viewer-side substrate, the first transparent protectivefilm of the viewer-side polarizing plate, the polarizing membrane of theviewer-side polarizing plate and the second transparent protective filmof the viewer-side polarizing plate is 0.6 mm or less.

In the present specification, the term “peaks” of the size distributionmeans local maximums in the size distribution curve, which can beobtained by classifying the particles according to the size (in units of0.1 μm) and plotting the size on the horizontal axis and the number ofparticles on the vertical axis.

The retardation value Re is defined by the formula (I), and theretardation value Rth is defined by the formula (II):Re=(nx−ny)×d  (I)Rth={(nx+ny)/2−nz}×d  (II)in which nx is a refractive index along the slow axis (the maximumrefractive index) in the film plane, ny is a refractive index in thedirection perpendicular to the slow axis in the film plane, nz is arefractive index along the depth of the film, and d is the thickness ofthe film in terms of nm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view schematically showing a structure of a liquidcrystal display of OCB mode having a polarizing plate.

FIG. 2 is a sectional view schematically showing a structure of a liquidcrystal display of VA mode having a polarizing plate.

DETAILED DESCRIPTION OF INVENTION

(Basic Structure of Liquid Crystal Display)

FIG. 1 is a sectional view schematically showing a structure of a liquidcrystal display of OCB mode having a polarizing plate.

The liquid crystal display shown in FIG. 1 comprises a backlight unit(1), a backlight-side polarizing plate (2), a liquid crystal cell of OCBmode (3) and a viewer-side polarizing plate (4).

The backlight unit (1) comprises a light source (11) and a light-guideplate (12). A diffusing plate or a film for increasing brightness may beprovided between the backlight unit (1) and the backlight-sidepolarizing plate (2).

The backlight-side polarizing plate (2) comprises a second transparentprotective film (21), a polarizing membrane (22), a first transparentprotective film of cellulose acetate film (23), an orientation layer(24) and an optically anisotropic layer (25) formed from liquid crystalcompound, layered in this order. The first transparent protective film(23) may have optical anisotropy. In FIG. 1, the arrow in theorientation layer (24) indicates a rubbing direction. In the opticallyanisotropic layer (25), molecules of the discotic liquid crystalcompound (251) are oriented in hybrid alignment, in which the liquidcrystal molecules near the orientation layer (24) are aligned at smallinclined angles while those far from the orientation layer (24) are atlarge inclined angles.

The liquid crystal cell of OCB mode (3) comprises a lower glasssubstrate (31), a lower transparent electro-conductive membrane (32), alower orientation layer (33), a liquid crystal layer (34), an upperorientation layer (35), an upper transparent electro-conductive membrane(36), a color filter (37) and an upper glass substrate (38), layered inthis order. In FIG. 1, the arrows in the orientation layers (33, 35)indicate rubbing directions. In the liquid crystal layer (34), moleculesof the rod-like liquid crystal compound (341) are oriented in bendalignment, in which the upper and lower molecules are symmetricallyaligned. The color filter (37) comprises blue parts (371), green parts(372), red parts (373) and black matrixes (374) among the parts.

The viewer-side polarizing plate (4) comprises an optically anisotropiclayer (41) formed from liquid crystal compound, an orientation layer(42), a first transparent protective film of cellulose acetate film(43), a polarizing membrane (44), a second transparent protective film(45) and a light-diffusing layer (46), layered in this order. In theoptically anisotropic layer (41), molecules of the discotic liquidcrystal compound (411) are oriented in hybrid alignment, in which theliquid crystal molecules near the orientation layer (42) are aligned atsmall inclined angles while those far from the orientation layer (42)are at large inclined angles. In FIG. 1, the arrow in the orientationlayer (42) indicates a rubbing direction. The first transparentprotective film (43) has optical anisotropy, and the second transparentprotective film (45) may have optical isotropy. The light-diffusinglayer (46) comprises transparent resin, therein-dispersed firsttransparent fine particles (461) and second ones (462). The first andsecond transparent fine particles preferably have different refractiveindexes and different sizes (namely, the total size distribution curvepreferably has two peaks). They may be the same kind of particles (theymay have the same refractive index) but have different sizes, orotherwise they may have almost the same sizes (namely, the sizedistribution curve need not have clearly separated peaks) but havedifferent refractive indexes. It is also possible to use only one kindof particles. On the light-diffusing layer (46), a low-refractive indexlayer may be provided.

The distance (d1 in FIG. 1) between the color filter (37) and thelight-diffusing layer (46) is preferably 0.6 mm or less.

FIG. 2 is a sectional view schematically showing a structure of a liquidcrystal display of VA mode having a polarizing plate.

The liquid crystal display shown in FIG. 2 comprises a backlight unit(1), a backlight-side polarizing plate (2), a liquid crystal cell of VAmode (3), and a viewer-side polarizing plate (4).

The backlight unit (1) comprises a light source (11) and a light-guideplate (12). A diffusing plate and/or a film for increasing brightnessmay be provided between the backlight unit (1) and the backlight-sidepolarizing plate (2).

The backlight-side polarizing plate (2) comprises a second transparentprotective film (21), a polarizing membrane (22), and a firsttransparent protective film of cellulose acetate film (23), layered inthis order. The first transparent protective film (23) may have opticalanisotropy.

The liquid crystal cell of VA mode (3) comprises a lower glass substrate(31), a lower transparent electro-conductive membrane (32), a lowerorientation layer (33), a liquid crystal layer (34), an upperorientation layer (35), an upper transparent electro-conductive membrane(36), a color filter (37) and an upper glass substrate (38), layered inthis order. In FIG. 2, the arrows in the orientation layers (33, 35)indicate rubbing directions. In the liquid crystal layer (34), moleculesof the rod-like liquid crystal compound (341) are oriented so that theupper and lower molecules are symmetrically aligned and those at thecentral part are essentially vertically aligned. The color filter (37)comprises blue parts (371), green parts (372), red parts (373) and blackmatrixes (374) among the parts.

The viewer-side polarizing plate (4) comprises a first transparentprotective film of cellulose acetate film (43), a polarizing membrane(44), a second transparent protective film (45) and a light-diffusinglayer (46), layered in this order. The first transparent protective film(43) has optical anisotropy, and the second transparent protective film(45) may have optical isotropy. The light-diffusing layer (46) comprisestransparent resin, therein-dispersed first transparent fine particles(461) and second ones (462). The first and second transparent fineparticles preferably have different refractive indexes and differentsizes (namely, the total size distribution curve preferably has twopeaks). They may be the same kind of particles (they may have the samerefractive index) but have different sizes, or otherwise they may havealmost the same sizes (namely, the size distribution curve need not haveclearly separated peaks) but have different refractive indexes. It isalso possible to use only one kind of particles. On the light-diffusinglayer (46), a low-refractive index layer may be provided.

The distance (d2 in FIG. 2) between the color filter (37) and thelight-diffusing layer (46) is preferably 0.6 mm or less.

(Cellulose Acetate Film)

In the present invention, two cellulose acetate films are preferablyused as the two (first and second) transparent protective films of thepolarizing plate.

The cellulose acetate preferably has an acetic acid content in the rangeof 59.0 to 61.5%. The term “acetic acid content” means the amount ofcombined acetic acid per one unit weight of cellulose. The acetic acidcontent can be determined according to ASTM: D-817-91 (tests ofcellulose acetate).

The cellulose acetate has a viscosity average polymerization degree (DP)of preferably 250 or more, more preferably 290 or more.

Further, it is also preferred for the cellulose acetate to have a narrowmolecular weight distribution of Mm/Mn (Mm and Mn are weight and numberaverage molecular weights, respectively), which is determined by gelpermeation chromatography. The value of Mm/Mn is preferably in the rangeof 1.00 to 1.70, more preferably in the range of 1.30 to 1.65, mostpreferably in the range of 1.40 to 1.60.

Generally in preparing cellulose acetate, hydroxyl groups at 2-, 3- and6-position of cellulose unit are not equally substituted, and thesubstitution degree at 6-position is apt to be relatively small. In thecellulose acetate used in the invention, however, the substitutiondegree at 6-position is preferably not smaller than those at 2- and3-positions.

The hydroxyl group at 6-position is substituted in an amount ofpreferably 32% or more, more preferably 33% or more, most preferably 34%or more, based on the total substitution degree at 2-, 3- and6-positions. Further, the substitution degree at 6-position ispreferably 0.88 or more.

The cellulose acetate films used in the invention can be made ofcellulose acetate prepared according to the methods described inJapanese Patent Provisional Publication No. 11(1999)-5851 (Synthesisexamples 1 to 3).

The cellulose acetate film has a thickness of preferably 10 to 70 μm,more preferably 20 to 70 μm, most preferably 20 to 60 μm. The celluloseacetate film has a modulus of elasticity preferably in the range of3,000 Mpa or less, more preferably in the range of 2,500 Mpa or less.The film also preferably has a moisture-swelling coefficient of30×10⁻⁵/cm²/% RH or less. The moisture-swelling coefficient is morepreferably 15×10⁻⁵/cm²/% RH or less, most preferably 10×10⁻⁵/cm²/% RH orless. The moisture-swelling coefficient indicates how long a film sampleexpands when the relative humidity increases under a constanttemperature. The film is preferably stretched biaxially so that thepolymer molecules may be aligned in the plane, and thereby thedistortion induced by change of temperature and humidity can beeffectively reduced.

The cellulose acetate film has a surface roughness (Ra) of preferably0.2 μm or less, more preferably 0.16 μm or less, most preferably 0.14 μmor less. It is particularly effective to control the surface roughness(Ra) of the light-diffusing layer-side surface of the second transparentprotective film. Generally in preparing a cellulose acetate film,relatively fine wrinkles are apt to be formed laterally. The surfaceroughness, therefore, is measured along the lateral direction at arandom position to estimate roughness per 100 mm. The cut-off value isset at 0.8 mm.

The cellulose acetate film is preferably prepared according to a solventcast method. In the solvent cast method, a solution (dope) in whichcellulose acetate is dissolved in an organic solvent is used.

The organic solvent preferably contains a solvent selected from thegroup consisting of an ether having 2 to 12 carbon atoms, a ketonehaving 3 to 12 carbon atoms, an ester having 2 to 12 carbon atoms and ahalogenated hydrocarbon having 1 to 6 carbon atoms.

The ether, the ketone or the ester may have a cyclic structure. Acompound having two or more functional groups of ether, ketone or ester(—O—, —CO— or —COO—) is also usable as the solvent. The organic solventmay have other functional groups such as alcoholic hydroxyl.

Examples of the ether include diisopropyl ether, dimethoxymethane,dimethoxyethane, 1,4-dioxane, 1,3-dioxolan, tetrahydrofuran, anisole andphenetole.

Examples of the ketone include acetone, methyl ethyl ketone, diethylketone, diisobutyl ketone, cyclohexanone and methylcyclohexanone.

Examples of the ester include ethyl formate, propyl formate, pentylformate, methyl acetate, ethyl acetate and pentyl acetate.

Examples of the compound having two or more functional groups include2-ethoxyethyl acetate, 2-methoxyethanol and 2-butoxyethanol.

The halogenated hydrocarbon preferably has one or two carbon atoms, morepreferably one carbon atom. The halogen is preferably chlorine. Thehydrogen in the halogenated hydrocarbon is substituted with halogen inan amount of preferably 25 to 75 mol. %, more preferably 30 to 70 mol.%, further preferably 35 to 65 mol. %, most preferably 40 to 60 mol. %.A typical halogenated hydrocarbon is methylene chloride.

Two or more kinds of the solvents may be mixed to use in combination.

The cellulose acetate solution can be prepared in an ordinary manner.The term “ordinary manner” means that the preparation is carried out ata temperature of 0° C. or more (room temperature or elevatedtemperature). The cellulose acetate solution (dope) can be preparedthrough a common process by means of a common apparatus in the normalsolvent cast method. In the normal process, a halogenated hydrocarbon(particularly, methylene chloride) is preferably used as the solvent.

The amount of cellulose acetate in the solution is preferably in therange of 10 to 40 wt. %, more preferably in the range of 10 to 30 wt. %.To the organic (main) solvent, additives described after may beoptionally added.

Cellulose acetate and the organic solvent are mixed and stirred at roomtemperature (0 to 40° C.) to prepare the solution. For preparing thesolution in high concentration, the preparation may be carried out at anelevated temperature under a high pressure. In that case, the celluloseacetate and the organic solvent are placed in a vessel resistingpressure. After the vessel is sealed, the mixture is stirred under anincreased pressure at an elevated temperature. The temperature iscontrolled so that it may be higher than the boiling point of thesolvent at atmospheric pressure but so that the solvent may not boil.The temperature is normally in the range of 40° C. or more, preferablyin the range of 60 to 200° C., more preferably in the range of 80 to110° C.

Before placed in the vessel, the components of the solution may bebeforehand mixed. They may be also added one by one into the vessel. Thevessel must be equipped with a stirring means. Inactive gas such asnitrogen gas may be charged in the vessel to increase the innerpressure. Otherwise, the vessel may be heated to elevate the vaporpressure of the solvent so that the inner pressure may increase. Afterthe vessel is sealed, each component may be added under an elevatedpressure.

The vessel is preferably heated from outside. For example, a jacketheater is preferably used. Otherwise, liquid heated with a plate heaterplaced outside of the vessel may be made to flow through a pipe woundaround the vessel, to heat the whole vessel.

The mixture is preferably stirred with a propeller mixer provided in thevessel. The wing of the propeller preferably has a length reaching theinside wall of the vessel. Further, at the tip of the wing, a scratchingmean is provided to scratch and renew liquid attached on the insidewall.

In the vessel, various meters such as pressure gauge and thermometer maybe provided. The components are dissolved in the solvent in the vessel.The thus-prepared dope may be cooled and then taken out of the vessel,or may be taken out and then cooled with a heat exchanger.

The solution can be prepared according to the cooling dissolutionmethod, which makes it possible to dissolve cellulose acetate in anorganic solvent in which cellulose acetate cannot be dissolved by aconventional process. Further, according to that method, celluloseacetate can be rapidly and homogeneously dissolved in an organic solventin which cellulose acetate can be dissolved by a conventional process.

First in the process of cooling dissolution method, cellulose acetate isgradually added with stirring into an organic solvent at roomtemperature.

The amount of cellulose acetate in the mixture is preferably in therange of 10 to 40 wt. %, more preferably in the range of 10 to 30 wt. %.Various additives described after may be added in the mixture.

The prepared mixture is cooled to a temperature of −100 to −10° C.(preferably −80 to −10° C., more preferably −50 to −20° C., mostpreferably −50 to −30° C.). The cooling procedure can be carried out,for example, with dry icemethanol bath (−75° C.) or with cooled ethyleneglycol solution (−30 to −20° C.). Through the cooling procedure, themixture is solidified.

The cooling rate is preferably 4° C./minute or more, more preferably 8°C./minute or more, and most preferably 12° C./minute or more. Thecooling rate is preferably as fast as possible. However, a theoreticalupper limit of the cooling rate is 10,000° C. per second, a technicalupper limit is 1,000° C. per second, and a practical upper limit is 100°C. per second. The cooling rate means the change of temperature at thecooling step per the time taken to complete the cooling step. The changeof temperature means the difference between the temperature at which thecooling step is started and the temperature at which the cooling step iscompleted.

The cooled mixture is then warmed to a temperature of 0 to 200° C.(preferably 0 to 150° C., more preferably 0 to 120° C., most preferably0 to 50° C.). Through the warming procedure, cellulose acetate isdissolved in the organic solvent. For warming, the mixture may be leftat room temperature or may be heated in a warm bath.

The warming rate is 4° C./minute or more, more preferably 8° C./minuteor more, and most preferably 12° C./minute or more. The warming rate ispreferably as fast as possible. However, a theoretical upper limit ofthe cooling rate is 10,000° C. per second, a technical upper limit is1,000° C. per second, and a practical upper limit is 100° C. per second.The warming rate means the change of temperature at the warming step perthe time taken to complete the warming step. The change of temperaturemeans the difference between the temperature at which the warming stepis started and the temperature at which the warming step is completed.

Thus, a homogeneous solution can be prepared. If cellulose acetate isnot sufficiently dissolved, the cooling and warming procedures may berepeated. It can be judged by observation with the eyes whethercellulose acetate is sufficiently dissolved or not.

In the process of cooling dissolution method, a sealed vessel ispreferably used to prevent contamination of water, which may be causedby dew condensation at the cooling step. Further, the mixture may becooled under a reduced pressure so that the time taken to complete thecooling step can be shortened, and hence a vessel resisting pressure ispreferably used to conduct the procedures under a reduced pressure.

According to differential scanning calorimetric measurement (DSC), a 20wt. % solution prepared by dissolving cellulose acetate (acetic acidcontent: 60.9%, viscosity average polymerization degree: 299) in methylacetate through the cooling dissolution process has a pseudo-phasetransition point between gel and sol at about 33° C. Below thattemperature, the solution is in the form of homogeneous gel. Thesolution, therefore, must be kept at a temperature above thepseudo-phase transition point, preferably at a temperature higher thanthe pseudo-gel phase transition point by about 10° C. The pseudo-gelphase transition point depends upon various conditions such as theorganic solvent, the acetic acid content, the viscosity averagepolymerization degree and the concentration of cellulose acetate.

The cellulose acetate film is formed from the prepared cellulose acetatesolution (dope) according to the solvent cast method.

The dope is cast on a drum or a band, and the solvent is evaporated toform a film. The solid content of the dope is preferably controlled inthe range of 18 to 35%. The surface of the drum or band is preferablybeforehand polished to be a mirror. The casting and drying steps of thecasting method are described in U.S. Pat. Nos. 2,336,310, 2,367,603,2,492,078, 2,492,977, 2,492,978, 2,607,704, 2,739,069, 2,739,070,British Patent Nos. 640,731, 736,892, Japanese Patent Publication Nos.45(1970)-4554, 49(1974)-5614, Japanese Patent Provisional PublicationNos. 60(1985)-176834, 60(1985)-203430 and 62(1987)-115035.

The surface temperature of the drum or band is preferably 10° C. orbelow. After cast on the drum or band, the dope is blown with air for 2seconds or more to dry. The formed film is then peeled, and blown withhot air whose temperature is successively changed from 100° C. to 160°C. in order to evaporate remaining solvent. This procedure is describedin Japanese Patent Publication No. 5(1993)-17844. That procedure canshorten the time taken to complete the steps of cooling to peeling. Forperforming the procedure, the cast dope must gel at the surfacetemperature of the drum or band.

Two or more cellulose acetate solutions (dopes) can be cooperativelycast to form two or more layers. For example, two or more outlets arearranged at intervals along the running direction of the support, andfrom each outlet each cellulose acetate solution is cast to form alayered film (Japanese Patent Provisional Publication Nos.61(1986)-158414, 1(1989)-122419 and 11(1999)-198285). Otherwise,cellulose acetate solutions may be cast from two outlets to form a film(Japanese Patent Publication No. 60(1985)-27562, Japanese PatentProvisional Publication Nos. 61(1986)-94724, 61(1986)-947245,61(1986)-104813, 61(1986)-158413 and 6(1994)-134933). Further, a flow ofhigh-viscous cellulose acetate solution may be enclosed with a flow oflow-viscous one to form a layered flow, and the high- and low-viscoussolutions in the layered flow may be simultaneously extruded to producea film (Japanese Patent Provisional Publication No. 56(1981)-162617).

Further, Japanese Patent Publication No. 44(1969)-20235 disclosesanother film preparation. In the disclosed process, a cellulose acetatesolution is cast on the support from one outlet to form a film. Afterpeeled from the support, the formed film is turned over and again placedon the support. On the thus appearing surface (having been in contactwith the support), another cellulose acetate solution is cast fromanother outlet to form a film.

The plural cellulose acetate solutions may be the same or different fromeach other. The function of each cellulose acetate layer can be given byeach corresponding solution extruded from each outlet.

Coating solutions for forming other functional layers (e.g., adhesivelayer, dye layer, antistatic layer, anti-halation layer, UV absorbinglayer, polarizing layer) can be simultaneously extruded with thecellulose acetate solutions.

In a conventional single layer preparation process, it is necessary toextrude a cellulose acetate solution having such high concentration andsuch high viscosity that the resultant film may have the aimedthickness. In that case, the cellulose acetate solution is often sounstable that solid contents are deposited to cause troubles and toimpair a plane surface. To avoid the problem, plural concentratedcellulose acetate solutions are simultaneously extruded from outletsonto the support. The thus-prepared thick film has an excellently planesurface. In addition, since the concentrated solutions are used, thefilm is so easily dried that the productivity (particularly, productionspeed) can be improved.

In the cellulose acetate solution, a plasticizer can be added to enhancemechanical strength of the resultant film or to shorten the time fordrying. The plasticizer is, for example, a phosphoric ester or acarboxylic ester. Examples of the phosphoric ester include triphenylphosphate (TPP) and tricresyl phosphate (TCP). Typical examples of thecarboxylic ester are phthalate esters and citrate esters. Examples ofthe phthalate esters include dimethyl phthalate (DMP), diethyl phthalate(DEP), dibutyl phthalate (DBP), dioctyl phthalate (DOP), diphenylphthalate (DPP) and diethyl hexyl phthalate (DEHP). Examples of thecitrate esters include triethyl o-acetylcitrate (OACTE) and tributylo-acetylcitrate (OACTB). Further, butyl oleate, methylacetyl ricinolate,dibutyl sebacate and various trimellitic esters can be used. Theplasticizers of phosphate esters (DMP, DEP, DBP, DOP, DPP, DEHP) arepreferred. Particularly preferred are DEP and DPP.

The content of the plasticizer is preferably in the range of 0.1 to 25wt. %, more preferably in the range of 1 to 20 wt. %, and mostpreferably in the range of 3 to 15 wt. % based on the amount ofcellulose ester.

Further, a deterioration inhibitor (e.g., oxidation inhibitor, peroxidedecomposer, radical inhibitor, metal inactivating agent, oxygenscavenger, amine) may be incorporated in the cellulose acetate film. Thedeterioration inhibitor is described in Japanese Patent ProvisionalPublication Nos. 3(1991)-199201, 5(1993)-1907073, 5(1993)-194789,5(1993)-271471 and 6(1994)-107854. The content of the deteriorationinhibitor is preferably in the range of 0.01 to 1 wt. %, more preferablyin the range of 0.01 to 0.2 wt. % based on the amount of the dope. Ifthe content is less than 0.01 wt. %, the deterioration inhibitor giveslittle effect. If it is more than 1 wt. %, the inhibitor often oozes out(bleeds out) to appear on the surface of the film. Particularlypreferred deterioration inhibitors are butylated hydroxytoluene (BHT)and tribenzylamine (TBA).

A cellulose acetate film used as the first transparent protective filmhas optical anisotropy. As the second protective film, an opticallyisotropic cellulose acetate film can be used.

Two or more optically anisotropic cellulose acetate films may be used inthe liquid crystal display. For example, in addition to the polarizingplate of the invention (viewer-side polarizing plate), an opticallyanisotropic cellulose acetate film can be used in the backlight-sidepolarizing plate.

The above “cellulose acetate film used as the first transparentprotective film has optical anisotropy” means that the film has Re andRth retardation values in the ranges of 20 to 70 nm and 100 to 500 nm,respectively.

In the case where two optically anisotropic cellulose acetate films areused in the liquid crystal display, one film preferably has Re and Rthretardation values in the ranges of 20 to 70 nm and 100 to 250 nm,respectively.

On the other hand, in the case where one optically anisotropic celluloseacetate film is used in the display, it preferably has Re and Rthretardation values in the ranges of 40 to 150 nm and 200 to 500 nm,respectively.

The birefringence (Δn: nx−ny) of the cellulose acetate film ispreferably in the range of 0.001 to 0.002, and that along the thicknessof the film {(nx+ny)/2−nz} is preferably in the range of 0.001 to 0.04.

The optically anisotropic cellulose acetate film is prepared while itsoptically anisotropy is controlled by adding a retardation-increasingagent and/or by adjusting the production conditions (in particular,stretching conditions).

The retardation-increasing agent is preferably an aromatic compoundhaving at least two aromatic rings. The aromatic compound is used in anamount of preferably 0.01 to 20 weight parts, more preferably 0.05 to 15weight parts, most preferably 0.1 to 10 weight parts, based on 100weight parts of cellulose acetate. Two or more aromatic compounds may beused in combination.

The aromatic ring in the aromatic compound may be an aromatichydrocarbon ring or an aromatic heterocyclic ring. The molecular weightof the retardation-increasing agent is preferably in the range of 300 to800.

The retardation-increasing agents are described in Japanese PatentProvisional Publication Nos. 2000-111914, 2000-275434, 2001-166144 andPCT Publication No. 00/02619.

In the case where the retardation is controlled by stretching, thestretching ratio is preferably in the range of 3 to 100%.

The stretching can be carried out by means of a tenter. In thestretching process, a film formed by casting is peeled and immediatelystretched with the tenter so as to control the retardation. In the latestage of stretching, the film is preferably kept near the glasstransition temperature and narrowed to be balanced with the peelingspeed so that the standard deviation of slow axis angle may be small.

The film may be longitudinally stretched between rolls. In that case, ifthe distance between the rolls is widened, the standard deviation ofslow axis can be reduced.

The cellulose acetate film is preferably subjected to surface treatment.Examples of the surface treatment include corona discharge treatment,glow discharge treatment, flame treatment, acid treatment, alkalitreatment, and ultraviolet (UV) treatment. Further, in place of or inaddition to the surface treatment, an undercoating layer (described inJapanese Patent Provisional Publication No. 7(1995)-333433) may beprovided.

The surface treatments are carried out preferably at a temperature nothigher than Tg (glass transition temperature) of the film to improve theplane surface of the film. That is not higher than 150° C.

The surface energy of the film subjected to the surface treatment ispreferably not less than 55 mN/m, more preferably in the range of 60 to75 mN/m.

The surface energy can be measured by the contact angle method, the wetheating method or the adsorption method. These methods are described in“The basic theory and application of wetting (written in Japanese)”,published by Realize Co., Ltd, 1989. The contact angle method ispreferred. In that method, two solutions having known surface energiesare dropped onto the film. The contact angle of each drop is measured,and the surface energy of the film is calculated from the measuredcontact angles. The contact angle is, by definition, an angle (includingthe drop) between the film surface and the tangent of the drop surfaceat the crossing point.

The cellulose acetate film is preferably subjected to the acid or alkali(namely, saponifying) treatment, so as to enhance adhesion to thepolarizing plate.

It is particularly preferred to perform the alkali treatment.

As the alkali treatment, the steps of immersing the film surface in analkaline solution, neutralizing with an acidic solution, washing withwater and drying are preferably circularly carried out.

Examples of the alkaline solution include aqueous solutions of KOH andNaOH. The normality of hydroxyl ion is preferably in the range of 0.1 to3.0 N, more preferably in the range of 0.5 to 2.0 N. The temperature ofthe solution is preferably in the range of room temperature to 90° C.,more preferably in the range of 40 to 70° C.

The alkaline solution may be applied on the film surface in place ofimmersing in consideration of productivity. In that case, after thealkaline solution is applied to saponify the film surface, the film waswashed with water to remove the solution. As the solvent for the coatingsolution, alcohols (e.g., isopropyl alcohol, n-butanol, methanol,ethanol) are preferred in consideration of wettability. Aids fordissolving alkali (e.g., water, propylene glycol, ethylene glycol) maybe added.

The productivity of the polarizing plate is affected by moisturepermeability of the cellulose acetate film used as the transparentprotective film. The polarizing membrane and the protective film aregenerally laminated with an aqueous adhesive, which is gradually driedaccording as a solvent of the adhesive is diffused into the protectivefilm. If the film has high moisture permeability, the solvent is sorapidly dried that the productivity is improved. However, if thepermeability is too high, moisture in air comes into the polarizingmembrane to impair the polarizability when the liquid crystal display isused under some conditions (e.g., under humid conditions).

The moisture permeability of the cellulose acetate film is in the rangeof preferably 100 to 1,000 g/m²·24 hrs, more preferably 300 to 700g/m²·24 hrs.

(Light-Diffusing Layer)

As shown in FIGS. 1 and 2, the light-diffusing layer preferablycomprises transparent resin and therein-dispersed two kinds oftransparent fine particles. For example, cross-linked polystyrene beads(mean particle size: 3.6 μm, refractive index: 1.61) are used as thefirst transparent fine particles, and silica fine particles (meanparticle size: 1.0 μm, refractive index: 1.51) are as the second ones.

The light-diffusing function of the layer is given by difference ofrefractive index between the transparent resin and the fine particles.The difference of refractive index is in the range of preferably 0.02 to0.15, more preferably 0.03 to 0.13, most preferably 0.04 to 0.10.

The first fine particles (relatively large particles) preferably have asize distribution in which the peak (mode) is positioned in the range of2.5 to 5.0 μm. The second particles (relatively small particles)preferably have a size distribution in which the peak (mode) is in therange of 0.5 to 2.0 μm. A preferred size distribution of the particlescan be easily obtained by mixing the two kinds of particles havingdifferent mode sizes.

The second transparent fine particles (relatively small particles)contribute to an optimal angular distribution of light scattering. Inorder to improve the image qualities (to improve downward viewingangle), it is necessary to diffuse incident light in some degree. Infact, the more the light is diffused, the more the viewing angle isimproved. However, for making an image seen frontally have enoughbrightness to ensure preferred image qualities, it is necessary to makethe transmittance as high as possible. If the mode of the sizedistribution is positioned at 0.5 μm or more, the amount of lightscattered backward is reduced to keep the brightness. On the other hand,if the mode is at 2.0 μm or less, the light is enough scattered toimprove the viewing angle. In the size distribution of the secondtransparent fine particles, the mode is positioned more preferably inthe range of 0.6 to 1.8 μm, most preferably in the range of 0.7 to 1.6μm.

The first transparent fine particles (relatively large particles)contribute to an optimal surface scattering. For improving the imagequalities, it is also important to scatter incident light on thedisplaying screen surface and thereby to prevent the screen fromreflecting surrounding scenes. Therefore, the size distribution of thefirst fine particles is controlled so that the mode may be positioned inthe range of 2.0 to 5.0 μm.

The smaller haze value the surface has, the more the fog of image isreduced and accordingly the clearer image the display gives. However, ifthe haze value is too low, the screen surface reflects surroundingscenes, and further glittering points (scintillations) are observed onthe screen. In contrast, if the haze value is too high, the displayedimage is whitened. Accordingly, the haze value on the surface (hs) ispreferably in the range of 0.5 to 30, more preferably in the range of 7to 20, most preferably in the range of 7 to 15.

In order to control the haze value on the surface, the surface of resinlayer is preferably roughened adequately by the first transparent fineparticles (relatively large particles). The haze value can be determinedby means of a measuring apparatus (HR-100, Murakami ShikisaiGijutsuKenkyujo Co., Ltd,) according to JIS-K-7105.

If the size of the first particles is 2.0 μm or less, the roughness isso small that the screen surface cannot scatter light enough to preventthe reflection of surrounding scenes. If the size is 5.0 μm or more, thesurface has enough roughness to prevent the reflection of surroundingbut the displayed image is remarkably whitened to impair the imagequalities. Accordingly, in the size distribution of the firsttransparent fine particles, the mode is positioned preferably in therange of 2.2 to 4.7 μm, more preferably in the range of 2.4 to 4.5 μm.

As described above, with respect to the size of the fine particles, themode size is more important than the average (mean) size. In the presentspecification, the term “mode size” means a size to which the largestnumber of particles belong when the particles are classified accordingto the size (in terms of 0.1 μm). Hereinafter (also in Examples), the“size” of the fine particles means the mode size.

The surface has an average surface roughness (Ra) of preferably 1.2 μmor less, more preferably 0.8 μm or less, most preferably 0.5 μm or less.

The haze value, particularly the internal haze value (which greatlycontributes to diffusion of transmitted light) of the light-diffusinglayer closely relates to improvement of the viewing angle.

The light-diffusing layer provided on the viewer-side surface of thepolarizing plate diffuses light emitted from the backlight, and therebythe viewing angle characters are improved. If the light is too diffused,the amount of light scattered backward increases and accordingly thebrightness of an image seen frontally is lowered. Further, the sharpnessof the image is also impaired. In view of that, the internal haze valueis in the range of preferably 30 to 80%, more preferably 35 to 70%, mostpreferably 40 to 60%.

There are some methods for elevating the internal haze value. Forexample, the content of particles having sizes of 0.5 to 1.5 μm may beincreased, or the layer may be thickened. It is also effective to useparticles having larger refractive-indexes.

Apart from the internal haze, the surface roughness is preferablycontrolled to obtain such adequate surface haze that the displayed imagecan be clearly seen. The internal and surface hazes cooperatively give atotal haze value preferably in the range of 40 to 90%, more preferablyin the range of 45 to 80%, most preferably in the range of 50 to 70%.

As the transparent fine particles, plastic beads are preferred. Theplastic beads are preferably made of material having high transparency,and the difference of refractive index between the material and thetransparent resin is preferably in the aforementioned range.

Examples of the material for the beads include acrylstyrene copolymer(refractive index: 1.55), melamine resin (refractive index: 1.57),cross-linked acrylic resin (refractive index: 1.49), polycarbonate(refractive index: 1.57), polystyrene (refractive index: 1.60),cross-linked polystyrene resin (refractive index: 1.61), and polyvinylchloride (refractive index: 1.60).

As the transparent particles, inorganic fine particles may be used.Examples of the inorganic fine particles include silica beads(refractive index: 1.44) and alumina beads (refractive index: 1.63).

The amount of the transparent fine particles is preferably in the rangeof 5 to 30 weight parts per 100 weight parts of the transparent resin.

The fine particles are liable to settle down in the resin composition(transparent resin). For preventing that, inorganic filler (e.g.,silica) may be added. However, the inorganic filler often impairs thetransparency of the layer. Accordingly, in order not to lower thetransparency, the inorganic filler consisting of grains having sizes of0.5 μm or less are preferably used in an amount of less than 0.1 wt. %based on the amount of the transparent resin.

As the transparent resin used in the light-diffusing layer, a resinhardened with ultraviolet ray or electron beam is preferably used. Athermosetting resin is also usable. The resin may contain athermoplastic resin and a solvent.

The transparent resin has a refractive index of preferably 1.50 to 2.00,more preferably 1.57 to 1.90, most preferably 1.64 to 1.80.

The transparent resin preferably comprises binder of a polymer having amain chain of saturated hydrocarbon or polyether. The main chain is morepreferably hydrocarbon, and the polymer is preferably cross-linked. Thepolymer having the main chain of saturated hydrocarbon is preferablyprepared from ethylenically unsaturated monomers through polymerizationreaction. For preparing the cross-linked polymer, monomers having two ormore ethylenically unsaturated groups are preferably used. Examples ofthe monomer having two or more ethylenically unsaturated groups includeesters of polyhydric alcohol and (meth)acrylic acid (e.g., ethyleneglycol di(meth)acrylate, 1,4-dichlorohexane diacrylate, pentaerythritoltetra(meth)acrylate, pentaerythritol tri(meth)acrylate,trimethylolpropane tri(meth)acrylate, trimethylolethanetri(meth)acrylate, dipentaerythritol tetra(meth)acrylate,dipentaerythritol penta(meth)acrylate, dipentaerythritolhexa(meth)acrylate, 1,3,5-cyclohexane-triol trimethacrylate,polyurethane polyacrylate, polyester polyacrylate), vinylbenzene andderivatives thereof (e.g., 1,4-divinylbenzene, 4-vinylbenzoicacid-2-acryloyl ethyl-ester, 1,4-divinylcyclohexanone), vinyl-sulfones(e.g., divinylsulfone), acrylamides (e.g., methylene bisacryl-amide) andmethacrylamide. In view of hardness and scratching resistance of thelayer, acrylate having five or more functional groups is preferred. Amixture of dipenta-erythritol pentaacrylate and dipenta-erythritolhexa-acrylate is commercially available and particularly preferablyused.

These monomers having ethylenically unsaturated groups are dissolved ina solvent together with various polymerization initiators and additives.The thus-prepared solution (coating solution) is applied on a support,dried and polymerized to harden by ionization radiation or heat.

In place of or in addition to the monomers having two or moreethylenically unsaturated groups, cross-linking groups may be introducedinto the binder to be cross-linked. Examples of the cross-linking groupinclude isocyanate group, epoxy group, aziridine group, oxazolidinegroup, aldehyde group, carbonyl group, hydrazine group, carboxyl group,methylol group, and active methylene group. Further, the cross-linkedstructure can be also obtained by monomers such as vinylsulfonic acid,acid anhydride, cyanoacrylate derivative, melamine, etherized methylol,ester, urethane, and metal alkoxide (e.g., tetramethoxy-silane).Further-more, the binder may be cross-linked by decomposition of somemonomers such as block isocyanate group. As the cross-linking group, notonly groups that immediately induce cross-linking reaction but alsogroups that are decomposed to cause the reaction can be used. The binderhaving the cross-linking group can be applied and cross-linked byheating.

Besides the above polymer, the transparent resin binder can comprise acopolymer of monomers having high refractive indexes and/or superfineparticles of metal oxide having a high refractive index.

Examples of the monomers having high refractive indexes includebis(4-methacryloylthiophenyl) sulfide, vinylnaphthalene, vinyl phenylsulfide, and 4-methacryloxyphenyl 4′-methoxyphenyl thioether.

The size of the superfine particles is preferably 100 nm or less, morepreferably 50 nm or less. The metal oxide having a high refractive indexis preferably an oxide of at least one metal selected from the groupconsisting of zirconium, titanium, aluminum, indium, zinc, tin andantimony. Examples of the metal oxide include ZrO₂, TiO₂, Al₂O₃, In₂O₃,ZnO, SnO₂, Sb₂O₃ and ITO. Among then, ZrO₂ is particularly preferred.The amount of the superfine particles is preferably in the range of 10to 90 wt. %, more preferably in the range of 20 to 80 wt. % based on thetotal weight of the transparent resin.

The light-diffusing layer is preferably formed on the cellulose acetatefilm through a coating process. Normally, a coating solution is directlyapplied on the second transparent protective film of cellulose acetateto form the diffusing layer. Otherwise, the layer is beforehand formedon another cellulose acetate film, which is then laminated on the secondprotective film.

In the case where the light-diffusing layer is formed on the celluloseacetate film, it is particularly preferred to use a mixed solvent in thecoating solution. The mixed solvent preferably comprises a solventdissolving cellulose acetate and another solvent not dissolving thecellulose acetate. Preferably, the solvent (or at least one of thesolvents) not dissolving the cellulose acetate has a higher boilingpoint than that (or at least one of those) dissolving the celluloseacetate. The boiling point of the not-dissolving solvent (the highestboiling point if two or more not-dissolving solvents are contained) ishigher than that of the dissolving solvent (than the lowest boilingpoint if two or more dissolving solvents are contained) preferably by30° C. or more, most preferably by 50° C. or more.

Examples of the solvent dissolving the cellulose acetate include ethershaving 2 to 12 carbon atoms (e.g., dibutyl ether, dimethoxy methane,dimethoxy ethane, diethoxy ethane, propylene oxide, 1,4-dioxane,1,3-dioxolane, 1,3,5-trioxane, teterahydrofuran, anisole, phenetole),ketones having 3 to 12 carbon atoms (e.g., acetone, methyl ethyl ketone,diethyl ketone, dipropyl ketone, diisobutyl ketone, cyclopentanone,cyclohexanone, methyl cyclohexanone), esters having 1 to 12 carbon atoms(e.g., ethyl formate, propyl formate, n-pentyl formate, methyl acetate,ethyl acetate, methyl propionate, ethyl propionate, n-pentyl acetate,γ-butyrolactone), organic solvents having two or more kinds offunctional groups (e.g., methyl 2-methoxyacetate, methyl2-ethoxyacetate, ethyl 2-ethoxyacetate, ethyl 2-ethoxypropionate,2-methoxyehanol, 2-propoxyethanol, 2-butoxyethanol,1,2-diacetoxyacetone, acetylacetone, diacetone alcohol, methylacetoacetate, ethyl acetoacetate). Two or more solvents can be used incombination.

Examples of the solvent not dissolving the cellulose acetate includealcohols (e.g., methanol, ethanol, 1-propanol, 2-propanol, 1-butanol,2-butanol, tert-butanol, 1-pentanol, 2-methyl-2-butanol, cyclohexanol),esters (e.g., isobutyl acetate), and ketones (e.g., methyl isobutylketone, 2-octanone, 2-pentanone, 2-hexanone, 2-heptanone, 3-pentanone,3-heptanone and 4-heptanone). Two or more solvents can be used incombination.

The weigh ratio (A/B) of the total amount of the solvent(s) dissolvingthe cellulose acetate (A) per that of the solvent(s) not dissolving (B)is preferably in the range of 5/95 to 50/50, more preferably in therange of 10/90 to 40/60, most preferably in the range of 15/85 to 30/70.

In forming the light-diffusing layer, the coating solution is appliedand hardened by irradiation of electron beams or ultraviolet rays.

In the irradiation of electron beams, electron beams emitted from anelectron accelerator can be used. The electron beams have energy in therange of 50 to 1,000 KeV, preferably in the range of 100 to 300 KeV.Examples of the electron accelerator include Cockcroft-Waltonaccelerator, Van de Graaff accelerator, resonant transformingaccelerator, insulating core-transforming accelerator, linearaccelerator, dinamitron, and radio-frequency accelerator. In theirradiation of ultraviolet rays, various light sources such as extrahigh pressure mercury lamp, high pressure mercury lamp, low pressuremercury lamp, carbon arc lamp, xenon arc lamp and metal halide arc lampcan be used.

The thickness of the light-diffusing layer is in the range of preferably0.5 to 50 μm, more preferably 1 to 20 μm, further preferably 2 to 10 μm,most preferably 3 to 7 μm.

(Low Refractive Index Layer)

A low refractive index layer can be provided on the light-diffusinglayer as the top surface layer, to give anti-reflection function to thepolarizing plate.

The low refractive index layer has a refractive index in the range of1.35 to 1.45.

The refractive index of the low refractive index layer preferablysatisfies the following formula:(mλ/4)×0.7<n ₁ ×d ₁<(mλ/4)×1.3in which m is a positive odd number (usually 1), n₁ is the refractiveindex of the low refractive index layer, d₁ is the thickness (nm) of thelow refractive index layer, and λ is a wavelength of visible light inthe region of 450 to 650 nm.

When the refractive index (n₁) satisfies the above formula, a certainpositive odd number (m) (which is usually 1) satisfying the formula canbe found in the above wavelength region.

The low refractive index layer is preferably made of afluorine-containing resin. In detail, it can be prepared by hardening athermosetting or ionization radiation-setting cross-linkablefluorine-containing compound. The hardened fluorine-containing resin hasa coefficient of kinetic friction preferably in the range of 0.03 to0.15, and gives a contact angle with water preferably in the range of90° to 120°.

Examples of the cross-linkable fluorine-containing compound include aperfluoroalkyl-containing silane compound (e.g.,(heptadecafluoro-1,1,2,2-tetradecyl)triethoxysilane) and afluorine-containing copolymer derived from fluorine-containing monomersand monomers introducing cross-linking groups.

Examples of the fluorine-containing monomers include fluoroolefins(e.g., fluoroethylene, vinylidene fluoride, tetrafluoroethylene,hexafluoroethylene, hexafluoropropylene,perfluoro-2,2-dimethyl-1,3-dioxole), partially or completely fluorinated(meth)acrylic alkyl ester derivatives (e.g., Biscoat 6FM [trade name,Osaka Organic Chemicals Co., Ltd.], M-2020 [trade name, Daikin Co.,Ltd.], and partially or completely fluorinated vinyl ethers.

Examples of the monomers introducing cross-linking groups include a(meth)acrylate monomer having a cross-linking group (e.g., glycidylmethacrylate), and a (meth)acrylate monomer having carboxyl, hydroxyl,amino or sulfonic acid group (e.g., (meth)acrylic acid, methylol(meth)acrylate, hydroxyalkyl (meth)acrylate, allylic acrylate). Afterthe (meth)acrylate monomers having carboxyl, hydroxyl, amino or sulfonicacid group are copolymerized, cross-linked structure can be formed inthe manner described in Japanese Patent Provisional Publication Nos.10(1998)-25388 and 10(1998)-147739.

As well as the copolymer derived from fluorine-containing monomers andmonomers introducing cross-linking groups, a copolymer derived fromthese monomers and other monomers can be also used.

The usable monomers other than the above monomers are not particularlyrestricted. Examples of them include olefins (e.g., ethylene, propylene,isoprene, vinyl chloride, vinylidene chloride), acrylate esters (e.g.,methyl acrylate, ethyl acrylate, 2-ethylhexyl acrylate), methacrylateesters (e.g., methyl methacrylate, ethyl methacrylate, butylmethacrylate, ethylene glycol dimethacrylate), styrene and derivativesthereof (e.g., divinylbenzene, vinyltoluene, α-methylstyrene), vinylethers (e.g., methylvinyl ether), vinyl esters (e.g., vinyl acetate,vinyl propionate, vinyl cinnamate), acrylamide derivatives (e.g.,N-tert-butylacrylamide, N-cyclohexyl-acrylamide), methacrylamidederivatives and acrylonitrile derivatives.

In the fluorine-containing resin, superfine particles of silicon oxideare preferably dispersed to make the layer tough against scratching. Themean size of the particles is preferably 0.1 μm or less, more preferablyin the range of 0.001 to 0.05 μm.

As the superfine particles of silicon oxide, commercially availablesilica sol can be directly added to a coating solution for forming thelow refractive index layer. Otherwise, various commercially availablesilica powders may be dispersed in an organic solvent to prepare asilicon oxide dispersion to be added into the coating solution.

(Optically Anisotropic Layer Formed from Liquid Crystal Compound)

The polarizing plate for OCB mode comprises an optically anisotropiclayer formed from liquid crystal compound.

The liquid crystal compounds include rod-like liquid crystal compoundsand discotic ones. The compound may be a polymer liquid crystalcompound. Further, a polymer in which liquid crystal molecules of lowmolecular weight are polymerized or cross-linked and thereby which nolonger behaves as liquid crystal is also usable.

Examples of the rod-like liquid crystal compound include azomethines,azoxys, cyanobiphenyls, cyanophenyl esters, benzoic esters,cyclohexanecarboxylate phenyl esters, cyanophenylcyclohexanes,cyano-substituted phenylpyrimidines, alkoxy-substitutedphenylpyrimidines, phenyldioxanes, tolanes, andalkenylcyclohexylbenzonitriles. Metal complexes are also included in therod-like liquid crystal compounds. Further, a liquid crystal polymer inwhich the repeating unit comprises a rod-like liquid crystal moiety isalso usable as the rod-like liquid crystal compound. In other words, therod-like liquid crystal compound may be combined with a (liquid crystal)polymer.

Descriptions of the rod-like liquid crystal compounds are found in“Kagaku-Sosetsu, Ekisho no Kageku” (written in Japanese), vol. 22(1994),Chapters 4, 7 and 11; “Ekisho Devise Handbook” (written in Japanese),chapter 3; and Japanese Patent publication No. 2000-304932.

It is particularly preferred to use discotic liquid crystal compounds.

Examples of the discotic liquid crystal compound include benzenederivatives described in C. Destrade et al., Mol. Cryst. vol. 71, pp.111, (1981); truxene derivatives described in C. Destrade et al., MolCryst. vol. 122, pp. 141. (1985), Physics lett. A, vol. 78, pp. 82,(1990); cyclohexane derivatives described in B. Kohn et al., Angew.Chem. vol. 96, pp. 70, (1984); and macrocyclic compounds ofazacrown-type or phenylacetylene-type described in J. M. Lehn et al., J.Chem. Commun. pp. 1794, (1985), and J. Zhang et al., J. Am. Chem. Soc.vol. 116, pp. 2655, (1994). The above discotic compound generally has astructure in which the discotic structure unit is located at the centeras a parent core and further straight chain groups such as alkyl, alkoxyand substituted benzoyloxy groups are radially substituted. As thediscotic liquid crystal compounds, however, any compound can be used solong as it has negative uniaxial property and orientation property.

The resultant optically anisotropic layer formed from the discoticliquid crystal compound does not need to contain the molecules of theliquid crystal compound. For example, some low molecular weight-discoticliquid crystal compounds having reactive groups are polymerized orcross-linked by heat or light to form polymers that no longer behave asliquid crystal. Such polymers can be also used in the invention.Preferred examples of the discotic liquid crystal compound are describedin Japanese Patent Provisional Publication No. 8(1996)-50206.

The optically anisotropic layer is preferably formed from the discoticliquid crystal compound. Molecules of the discotic liquid crystalcompound preferably have discotic planes inclined from a plane of thecellulose acetate film at angles varying according to the direction ofdepth of the layer (namely, the molecules are preferably oriented inhybrid alignment).

The above-described angle (inclined angle) of the discotic planegenerally increases or decreases with increase of distance in thedirection of depth from the bottom of the optically anisotropic layer(namely, from the surface of the cellulose acetate film). The inclinedangle preferably increases with increase of the distance. Further,examples of variation of the inclined angle include continuous increase,continuous decrease, intermittent increase, intermittent decrease,variation containing continuous increase and decrease, and intermittentvariation containing increase or decrease. The intermittent variationcontains an area where the inclined angle does not vary in the course ofthe thickness direction of the layer. The inclined angle preferablytotally increases or decreases in the layer, even if it does not vary inthe course. The inclined angle more preferably increases totally, and itis particularly preferred to increase continuously.

The optically anisotropic layer can be generally prepared by the stepsof: coating an orientation layer with a solution of the discotic liquidcrystal compound and additives (e.g., polymerizable monomer,photo-polymerization initiator) dissolved in a solvent, drying, heatingto a temperature for forming a discotic nematic phase, and cooling withthe oriented condition (discotic nematic phase) kept. The orientation ispreferably fixed by polymerization (e.g., by radiation of UV light). Thetransition temperature from discotic nematic phase to solid phase ispreferably in the range of 70 to 300° C., especially 70 to 170° C.

The inclined angle of the discotic plane on the cellulose acetate filmside can be generally controlled by selecting the discotic compound ormaterials of the orientation layer, or by selecting methods for therubbing treatment. On the other hand, for controlling the inclined angleof the discotic plane on the surface side (air side), the discoticcompound or additives (e.g., plasticizer, surface-active agent,polymerizable monomer or polymer) used together with the discoticcompound are properly selected. Further, the extent of variation of theinclined angle can be also controlled by the above selections. Theadditives such as the plasticizer, the surface-active agent and thepolymerizable monomer are preferably compatible with the discoticcompound. They may give variation of the inclined angle, but preferablydo not inhibit the discotic compound molecules from aligning.

Examples of the polymerizable monomer include compounds having vinyl,vinyloxy, acryloyl or methacryloyl groups. The polymerizable monomer ispreferably an acrylate having plural functional groups. The number ofthe functional groups is preferably three or more, more preferably fouror more, most preferably six. A particularly preferred example of theacrylate having six functional groups is dipentaerythritol hexaacrylate.Two or more kinds of monomers having different numbers of functionalgroups can be mixed to use in combination.

The polymerizable monomer is preferably used in the amount of 1 to 50wt. %, especially 5 to 30 wt. % based on the amount of the discoticcompound.

The optically anisotropic layer may contain a polymer, which ispreferably compatible with the discotic compound and which alsopreferably do not inhibit the discotic liquid crystal molecules fromaligning. The polymer may give variation of the inclined angle. As thepolymer, cellulose esters (e.g., cellulose acetate, celluloseacetatepropionate, hydroxypropylcellulose and cellulose acetatebutylate)are preferably used. The polymer is used in an amount of preferably 0.1to 10 wt. %, more preferably 0.1 to 8.0 wt. %, most preferably 0.1 to5.0 wt. % based on the amount of the discotic compound.

(Orientation Layer)

The molecules of the liquid crystal compound are aligned with anorientation layer.

The orientation layer is preferably a cross-linked polymer membranesubjected to the rubbing treatment. More preferably, the orientationlayer is made of cross-linked two polymers. As the polymers, not onlypolymers originally cross-linkable but also ones cross-linked withcross-linking agents can be used.

The polymers having functional groups can be cross-linked by light, heator pH variation to form the orientation layer. Otherwise, highlyreactive cross-linking agents may be used to introduce linking groupsinto the polymers, so as to form the orientation layer.

For cross-linking the polymer, a coating solution containing thecross-linkable polymer and, if needed, the cross-linking agent isapplied on the cellulose acetate film, and then the cross-linkingreaction is induced by light, heat or pH variation. However, as long asthe resultant liquid crystal display has enough durability, the reactionmay be caused at any stage from the step of coating the film with theorientation layer to the final step of producing the resultant display.

In consideration of orientation of the liquid crystal molecules in theoptically anisotropic layer on the orientation layer, the cross-linkingreaction is preferably caused after the liquid crystal molecules arealigned. In the case where the coating solution containing the polymerand the cross-linking agent is applied and heated to dry on thecellulose acetate film to form the orientation layer, the cross-linkingreaction generally proceeds while the solution is heated and dried. (Ifthe heating temperature is low, the reaction further proceeds when theliquid crystal compound is heated to the temperature for forming liquidcrystal phase to form the optically anisotropic layer.) After theapplied and dried layer is subjected to the rubbing treatment to form anorientation layer, another coating solution containing the liquidcrystal compound is applied and heated above the temperature for formingthe liquid crystal phase. The heated solution on the orientation layeris cooled to prepare the optically anisotropic layer.

Examples of the polymers used for the orientation layer includepolymethyl metacrylate, polyacrylic acid, polymethacrylic acid,polystyrene, polymaleinimide, gelatin, polyvinyl alcohol, denaturedpolyvinyl alcohol, poly(N-methylolacrylamide), polyvinyltoluene,chlorosulfonated polyethylene, nitrocellulose, polyvinyl chloride,chlorinated polyolefin, polyester (e.g., polycarbonate), polyimide,polyvinyl acetate, carboxymethylcellulose, polyethylene, andpolypropylene. Copolymers thereof are also usable. Examples of thecopolymers include acrylic acid/methacrylic acid copolymer,styrene/maleinimide copolymer, styrene/vinyltoluene copolymer, vinylacetate/vinyl chloride copolymer, and ethylene/vinyl acetate copolymer.Silane coupling agents may be used. Preferred examples are water-solublepolymers such as poly(N-methylolacrylamide), carboxymethylcellulose,gelatin, polyvinyl alcohol and denatured polyvinyl alcohol. Gelatin,polyvinyl alcohol and denatured polyvinyl alcohol are more preferred,and polyvinyl alcohol and denatured polyvinyl alcohol are particularlypreferred.

It is most preferred to use two kinds of polyvinyl alcohols or denaturedpolyvinyl alcohols having different polymerization degrees.

The saponification degree of the polyvinyl alcohol is in the range ofpreferably 70 to 100%, more preferably 80 to 100%, more preferably 85 to95%. The polymerization degree is preferably in the range of 100 to3,000. Examples of the denatured polyvinyl alcohol include polyvinylalcohols denatured by copolymerization (introduced denaturing group:COONa, Si(OX)₃, N(CH₃)₃.Cl, C₉H₁₉COO, SO₃Na, C₁₂H₂₅, etc.), by chaintransfer (introduced denaturing group: COONa, SH, C₁₂H₂₅, etc.) and byblock polymerization (introduced denaturing group: COOH, CONH₂, COOR,C₆H₅, etc.).

With respect to the denatured polyvinyl alcohols, Japanese PatentProvisional Publication No. 8(1996)-338913 describes in detail theirsyntheses, measurement of visible absorption spectra and methods fordetermining the ratios of introduced denaturing groups.

Non- or alkylthio-denatured polyvinyl alcohols having saponificationdegrees of 85 to 95% are particularly preferred.

Examples of the cross-linking agent include aldehydes (e.g.,formaldehyde, glyoxal, glutaraldehyde), N-methylol compounds (e.g.,dimethylol urea, methyloldimethylhydantoin), dioxane derivatives (e.g.,2,3-dihydroxy-dioxane), compounds that works when the carboxylic groupis activated (e.g., carbenium, 2-naphthalenesulfonate,1,1-bispyrrolidino-1-chloropyridinium,1-morpholinocarbonyl-3-(sulfonatoaminomethyl)), active vinyl compounds(e.g., 1,3,5-triacryloyl-hexahydro-s-triazine,bis-(vinylsulfone)methane,N,N′-methylenebis-[β-(vinylsulfonyl)propionamide], active halogencompounds (e.g., 2,4-dichloro-6-hydroxy-s-triazine), isooxazoles anddialdehyde starch. Two or more cross-linking agents may be used incombination. In consideration of productivity, reactive aldehydes arepreferred. Particularly preferred is glutaraldehyde.

The amount of the cross-linking agent is in the range of preferably lessthan 50 wt. %, more preferably 0.1 to 20 wt. %, most preferably 0.5 to15 wt. % based on the amount of the polymer. The amount of non-reactedcross-linking agent remaining in the orientation layer is preferably notmore than 1.0 wt. %, more preferably not more than 0.5 wt. % based onthe amount of the orientation layer.

The orientation layer can be formed by the steps of applying a coatingliquid containing the polymer (and the cross-linking agent) onto thecellulose acetate film, heating to dry (and to cross-link), andsubjecting to the rubbing treatment. The coating liquid is preferablyprepared from a mixed solvent of water and an organic solvent havingdefoaming character (e.g., methanol). In the mixed solvent, water iscontained in an amount of preferably 1 wt. % or more, more preferably 9wt. % or more.

The pH value of the coating liquid is preferably adjusted at an optimalvalue according to the used cross-linking agent. If glutaraldehyde isused as the cross-linking agent, the pH is preferably in the range of4.5 to 5.5, more preferably at 5.0.

As the coating method, known methods such as spin-coating, dip-coating,curtain-coating, extrusion-coating, bar-coating and E-type coating canbe adopted. The E-type coating method is particularly preferred. Thethickness of the layer is preferably in the range of 0.1 to 10 μm. Theapplied layer is dried at a temperature of preferably 20 to 110° C.,more preferably 60 to 100° C., most preferably 80 to 100° C. The timefor drying is in the range of preferably 1 minute to 36 hours, morepreferably 5 minutes to 30 minutes.

After the polymer layer is cross-linked, the surface of the layer issubjected to rubbing treatment. The rubbing treatment can be conductedin the manner adopted widely in aligning liquid crystal molecules ofconventional liquid crystal displays. The surface of the layer is rubbedwith paper, cloth (gauze, felt, nylon, polyester) or rubber along acertain direction, to give the aligning function. Generally, the layeris rubbed several times with cloth on which fibers having the samelength and thickness are provided.

(Polarizing Membrane)

Examples of the polarizing membrane include an iodine polarizingmembrane, a polyene polarizing membrane and a dichromatic dye polarizingmembrane. The iodine polarizing membrane and the dye polarizing membraneare generally prepared from polyvinyl alcohol films.

The polarizing membrane and the cellulose acetate film are placed sothat the slow axes of the film may be essentially parallel to thetransmission axis of the membrane.

(Liquid Crystal Display)

The polarizing plate is advantageously used in a liquid crystal displayof OCB mode or VA mode.

The liquid crystal display of OCB or VA mode comprises two polarizingplates and a liquid crystal cell provided between them. The liquidcrystal cell comprises a pair of electrode substrates and liquid crystalplaced between them. The polarizing plate of the invention is used asthat provided on the viewer side (displaying screen side). Thepolarizing plate is placed so that its light-diffusing layer may be onthe displaying screen side.

In the liquid crystal cell of OCB (bend alignment) mode, rod-like liquidcrystal molecules in upper part and ones in lower part are essentiallyreversely (symmetrically) aligned. Because of the symmetrical alignmentof the liquid crystal molecules, the liquid crystal cell of OCB mode hasself-optical compensatory function. A liquid crystal display having theliquid crystal cell of OCB mode is disclosed in U.S. Pat. Nos. 4,583,825and 5,410,422. The liquid crystal display of OCB mode has an advantageof responding rapidly.

In the liquid crystal cell of VA mode, rod-like liquid crystal moleculesare essentially vertically aligned while voltage is not applied.

The liquid crystal cell of VA mode include some types:

-   -   (1) a liquid crystal cell of VA mode in a narrow sense        (described in Japanese Patent Provisional Publication No.        2(1990)-176625), in which rod-like liquid crystal molecules are        essentially vertically aligned while voltage is not applied, and        the molecules are essentially horizontally aligned while voltage        is applied;    -   (2) a liquid crystal cell of MVA mode (described in SID97,        Digest of tech. Papers, 28(1997), 845), in which the VA mode is        modified to be multi-domain type so as to enlarge the viewing        angle;    -   (3) a liquid crystal cell of n-ASM mode (described in Nippon        Ekisho Toronkai [Liquid crystal forum of Japan], Digest of tech.        Papers (1998), 58-59), in which rod-like liquid crystal        molecules are essentially vertically aligned while voltage is        not applied, and the molecules are essentially oriented in        twisted multi-domain alignment while voltage is applied; and    -   (4) a liquid crystal cell of SURVAIVAL mode (published in LCD        international 98).

The polarizing plate of the invention is particularly effective whenused in a liquid crystal display that can give color images. The liquidcrystal display giving color images comprises a liquid crystal cellequipped with a col- or filter. The color filter is generally providedon the viewer-side glass plate of the liquid crystal cell.

On the displaying screen-side of the liquid crystal cell, the colorfilter is provided in the form of a matrix or dotted pattern. The colorfilter may have such uneven thickness that the thickness at each colorpart is differently designed so as to obtain a preferred contrast ratio(ref. Japanese Patent Provisional Publication No. 60(1985)-159823),because the contrast ratio (ratio of brightness) of images given by theliquid crystal display depends on the wavelength of transmitted lightand on the thickness of liquid crystal layer.

A transparent electrode for driving the liquid crystal can be providedon the color filter. It is also preferred to provide a blacklight-absorbing area (black matrix) among the color parts. In the casewhere a switching device of thin-film transistor is provided on thecounter electrode side, the black matrix may be provided to cover thedevice.

The black matrix can be made of metal (e.g., chromium) or metal compound(e.g., chromium oxide, chromium nitride). The black matrix may consistof plural layers. The thickness of the black matrix is so controlledthat the transmittance [log₁₀(amount of incident light/amount oftransmitted light)] may be in the range of 2 to 3. In general, the blackmatrix preferably has a thickness of 0.06 to 0.2 μm. The black matrixcan be formed according to photo-etching method or liftoff method.

In providing the color filter, green parts (whose unevenness is easilyrecognized by the eyes) are preferably formed first.

Each color part in the color filter is formed from a polymer matrixcontaining dye or pigment (preferably, dye). The polymer matrix can beprepared from a light-sensitive resin comprising an aqueous solution ofprotein (e.g., casein, glue, gelatin) and therein-added potassiumdichromate or ammonium dichromate. The light-sensitive resin maycomprise an acrylate resin and a photo-cross-linking agent. For example,the light-sensitive resin is applied by means of a whirler to form acoating layer having a predetermined thickness. The coating layer isexposed to light through a mask, and developed to form a desired reliefimage, which is then colored with an aqueous dye acidified by aceticacid.

For preparing the color filter, commercially available dyes can be used.Examples of the red dyes include Lanasn Red S-2GL (Sandoz Ltd.), IrganolRed VL (Ciba-Geigy), Kayanol Milling Red RS and Kayakalan Scarlet GL(Nippon Kayaku Co., Ltd.), and Suminol Level Vinol 3GP (SumitomoChemical Co., Ltd.). Examples of the green dyes include DiamillaBrilliant Green 6B (Mitsubishi Chemical Industries, Ltd.), IliganolYellow 4GLS (Ciba-Geigy), Sumifix Starx Blue BS 100% (Sumitomo ChemicalCo., Ltd.), Kayakalan Yellow GL143 (Nippon Kayaku Co., Ltd.), BrilliantIndoblue (Hoechst), and Suminoil Yellow MR (Sumitomo Chemical Co.,Ltd.). Examples of the blue dyes include Solofelterkis Blue BRL(Ciba-Geigy), Kayanol Milling Cyanine G (Nippon Kayaku Co., Ltd.) andMitsui Acid Milling Sky Blue FSE (Mitsui Toatsu Chemicals, Inc.). Two ormore dyes may be mixed to use (as two component-system dye). For dyeingthe filter layer, 0.5 to 2 wt. % aqueous solution of the dye isacidified with acetic acid in an amount of 1 to 3 wt. %, to prepare adye solution. The layer is dyed preferably while heated in a hot bath at50 to 70° C. The time for dyeing is preferably in the range of 5 to 20minutes.

EXAMPLE 1

(Preparation of Cellulose Acetate Film used as the First TransparentProtective Film)

The following components were poured into a mixing tank, and stirred andheated to dissolve each component. Thus, a cellulose acetate solutionwas prepared. Cellulose acetate solution Cellulose acetate having acetic80 weight parts acid content of 60.9% (linter) Cellulose acetate havingacetic 20 weight parts acid content of 60.8% (linter) Triphenylphosphate 7.8 weight parts Biphenyl diphenyl phosphate 3.9 weight partsMethylene chloride 300 weight parts Methanol 45 weight parts

Independently, 4 weight parts of cellulose acetate having acetic acidcontent of 60.9% (linter), 25 weight parts of the following retardationincreasing agent, 0.5 weight part of silica fine particles (mean size:20 nm), 80 weight parts of methylene chloride and 20 weight parts ofmethanol were poured into another mixing tank, and stirred and heated toprepare a retardation increasing agent solution.(Retardation-Increasing Agent)

The cellulose acetate solution (470 weight parts) and theretardation-increasing agent solution (30 weight parts) were mixed andstirred well to prepare a dope. The prepared dope contained theretardation-increasing agent in the amount of 6.2 weight parts based on100 weight parts of cellulose acetate.

The dope was cast on a band by means of a band-casting machine. Afterthe surface temperature of the dope on the band reached at 35° C., thedope was dried for 1 minute. When the solvent remaining in the formeddope film reached 45 wt. %, the film was peeled from the band. The filmwas conveyed to a tenter-stretching zone, where it laterally stretchedby 28% with a tenter at 140° C. The stretched film was then dried at140° C. for 10 minutes, and further dried at 130° C. for 20 minutes.Thus, a cellulose acetate film (thickness: 60 μm) containing theremaining solvent in the amount of 0.3 wt. % was prepared.

The optical characters of the prepared cellulose acetate film weremeasured at the wavelength of 550 nm by means of an ellipsometer (M−150,JASCO COORPORATION), and thereby it was found that the Re and Rth valueswere 35 nm and 175 nm, respectively.

The surface of the prepared cellulose acetate film was coated with 5ml/m² of 1.5 N potassium hydroxide solution (solvent: water/isopropylalcohol/propylene glycol=14/86/15 volume %), kept at 60° C. for 10seconds, washed with water to remove the potassium hydroxide, and dried.The surface energy of the thus-treated film was measured according tothe contact angle method, to find 60 mN/m.

Thus, a cellulose acetate film used as the first transparent protectivefilm was produced.

(Formation of Orientation Layer)

On the cellulose acetate film of the first transparent protective film,the following coating solution was applied in the amount of 28 ml/m² bymeans of a wire bar coater of #16. The applied solution was dried withhot air at 60° C. for 60 seconds, and then further dried with hot air at90° C. for 150 seconds.

The formed layer was then subjected to rubbing treatment in which therubbing direction was at the angle of 45° to the longitudinal directionof the cellulose acetate film, to form an orientation layer. Coatingsolution for orientation layer The following denatured polyvinyl alcohol10 weight parts Water 371 weight parts Methanol 119 weight partsGlutaric aldehyde (cross-linking agent) 0.5 weight part

(Formation of Optically Anisotropic Layer)

In 102 g of methyl ethyl ketone, 41.01 g of the following discoticliquid crystal compound, 4.06 g of ethylene oxide denaturedtrimethlolpropanetriacrylate (V#360, Osaka Organic Chemicals Co., Ltd.),0.68 g of cellulose acetate butyrate (CAB-551-0.2, Eastman Chemical),1.35 g of a photopolymerization initiator (Irgacure 907, Ciba-Geigy) and0.45 g of a sensitizer (Kayacure DETX, Nippon Kayaku Co., Ltd.) weredissolved to prepare a coating solution. The coating solution wasapplied on the orientation layer by means of a wire bar coater of #4,and then heated in a thermostat zone at 130° C. for 2 minutes to orientthe molecules of the discotic compound in hybrid alignment. The film wasirradiated at 100° C. for 0.4 second with ultraviolet rays emitted froma high-pressure mercury lamp of 1200 W/cm, to polymerize and fix themolecules of the discotic compound. Thus, an optically anisotropic layerwas formed.

The Re retardation value of the formed optically anisotropic layer wasmeasured at 550 nm, to find 42 nm. The average angle (inclined angle)between the discotic planes and the cellulose acetate film surface wasfound 30°.(Discotic Liquid Crystal Compound)

(Preparation of Cellulose Acetate Film used as the Second TransparentProtective Film)

The following components were mixed to prepare cellulose acetatesolutions used as dopes for forming inner and outer (surface) layers.Each solution was prepared according to the cooling dissolution method.In detail, the components were mixed to prepare each mixturecomposition, which was then left at room temperature for 3 hours to forman inhomogeneous gel solution. After cooled at −70° C. for 6 hours, thegel solution was heated to 50° C. and stirred to obtain each solution.Cellulose acetate solutions for inner layer for surface layer Celluloseacetate 100 weight parts 100 weight parts (acetic acid content: 59.5%)Triphenyl phosphate 7.8 weight parts 7.8 weight parts Biphenyl diphenylphosphate 2.0 weight parts 2.0 weight parts Methyl acetate 306 weightparts 327 weight parts Cyclohexanone 122 weight parts 131 weight partsMethanol 30.5 weight parts 32.7 weight parts Ethanol 30.5 weight parts32.7 weight parts Silica particles 1.0 weight part 1.0 weight part (meansize: 20 nm)

The obtained dope for outer layer was filtered at 50° C. through afilter paper (absolute filtration precision: 0.0025 mm, FH025 PALLCORPORATION). The dope for inner layer was filtered at 50° C. throughanother filter paper (absolute filtration precision: 0.01 mm, #63 TOYOROSHI KAISHA LTD.).

The dopes were cooperatively cast on a metal support from a three-layerco-casting die, so that the dope for inner layer might be sandwichedwith the dope for outer layer and also so that the dry thickness of theinner and outer layers might be 48 μm and 6 μm, respectively. Afterstepwise dried on the support at 70° C. for 3 minutes and at 140° C. for5 minutes, the formed film was peeled from the support. The peeled filmwas further dried at 130° C. for 30 minutes to evaporate the solvent.The amount of the solvent remaining in the film was 30 wt. % when thefilm was peeled from the support, while that was 0.9 wt. % when all theprocedures were completed.

The peeled film was uniaxially stretched by 10% in the lateral directionby means of a tenter, and further uniaxially stretched by 15% in thelongitudinal direction between rolls. The surfaces of the rolls forstretching were beforehand polished to be mirrors. Heated oil wascirculated to control the temperature of the rolls at 135° C. while thefilm was stretched. After stretched, the film was dried and wound up at130° C. for 30 minutes. The thickness of the resultant film was 50 μm.

The surface roughness (Ra) of the film per 100 mm was measured at tenpoints randomly selected in the lateral direction, to find 0.09 μm onaverage.

Thus, the cellulose acetate film used as the second transparentprotective film was produced.

(Formation of Light-Diffusing Layer)

A mixture of 13.8 weight parts of ultraviolet curable resin (DPHA,Nippon Kayaku Co., Ltd.; refractive index: 1.51), 42.0 weight parts ofultraviolet curable resin (KZ-7114A, JSR Co., Ltd.; refractive index:1.68), 7.7 weight parts of methyl iso-butyl ketone containingcross-linked polystyrene beads (SXS-350H, Soken Kagaku Co., Ltd.;particles size: 3.5 μm; refractive index: 1.61) dispersed in the amountof 30 wt. %, and 20 weight parts of methyl ethyl ketone containingsilica fine particles (MXS-150CF, Nippon Shoukubai Co., Ltd.) dispersedin the amount of 30 wt. % was prepared. To the mixture, 10.0 weightparts of methyl ethyl ketone and 1.9 weight parts of methyl iso-butylketone were added to prepare a coating solution.

The prepared coating solution was applied on the cellulose acetate filmof the second transparent protective film in the amount of 8.6 ml/m².The coating layer was dried, and then exposed to ultraviolet light(illuminance: 140 mW/cm², exposure: 300 mJ/cm²) emitted from anair-cooled metal halide lamp of 160 W/cm (Eyegraphics Co., Ltd.) toharden the layer.

Thus, a light-diffusing layer was prepared.

The haze of the second transparent protective film on which thediffusing layer was provided was determined by means of a measuringapparatus (HR-100, Murakami Shikisai Gijutsu-kenkyujo Co., Ltd,)according to JIS-K-7105, to be found 56%.

(Production of Viewer-Side Polarizing Plate for OCB Mode)

Iodine was adsorbed on a stretched polyvinyl alcohol film to prepare apolarizing membrane.

The second transparent protective film (on which the diffusing layer wasprovided) was saponified and laminated on one surface of the polarizingmembrane with polyvinyl alcohol adhesive, so that the second transparentprotective film (cellulose acetate film) might be contact with themembrane.

The first transparent protective film (on which the opticallyanisotropic layer was provided) was laminated on the other surface ofthe membrane with polyvinyl alcohol adhesive, so that the firsttransparent protective film (cellulose acetate film) might be contactwith the membrane. The first protective film was placed so that the slowaxis of the film might be parallel to the transmission axis of thepolarizing membrane.

Thus, a viewer-side polarizing plate for OCB mode was produced.

(Production of Backlight-Side Polarizing Plate)

Iodine was adsorbed on a stretched polyvinyl alcohol film to prepare apolarizing membrane.

The first transparent protective film (on which the opticallyanisotropic layer was provided) was laminated on one surface of themembrane with polyvinyl alcohol adhesive, so that the first transparentprotective film (cellulose acetate film) might be contact with themembrane. The first protective film was placed so that the slow axis ofthe film might be parallel to the transmission axis of the polarizingmembrane.

A commercially available cellulose triacetate film (Fujitac TD80, FujiPhoto Film Co., Ltd.) was saponified and laminated on the other surfaceof the polarizing membrane.

Thus, a backlight-side polarizing plate was produced.

(Production of Liquid Crystal Display of OCB Mode)

On a glass plate provided with an ITO electrode, a polyimide layer wasformed and the surface of the layer was subjected to rubbing treatmentto form an orientation layer. Further, another glass substrate havingorientation layer was prepared in the same manner. The thus-prepared twoglass plates were faced to each other so that the rubbing directionswere parallel to each other, and combined so that the gap between theplates might be 6 μm. A commercially available liquid crystal compound(Δn=0.1396; trade name: ZLI1132, Merck & Co., Inc.) was inserted intothe gap, to prepare a liquid crystal cell of OCB mode.

On one side of the prepared liquid crystal cell, the viewer-sidepolarizing plate was laminated. On the other side, the backlight-sidepolarizing plate was laminated. The viewer-side polarizing plate wasplaced so that the optically anisotropic layer might be contact with thecell and so that the rubbing direction of the anisotropic layer might beanti-parallel to that of the liquid crystal cell. On the backlight-sidepolarizing plate, a backlight unit was provided.

Voltage of a square wave (55 Hz) was applied to the liquid crystal cell.An image was displayed according to normally white mode (white: 2V,black: 5V). A ratio of contrast (white/black) was measured by means of ameter (EZ-Contrast 160D, ELDIM) at eight displaying states of L1 (fullblack) to L8 (full white).

The viewing angle was evaluated as an angle range giving a contrastratio of 10 or more without reversing black tones (between L1 and L2).As a result, it was found that the angle was 80° in each of the upward,downward and right-leftward directions.

EXAMPLE 2

(Production of Viewer-Side Polarizing Plate for VA Mode)

Iodine was adsorbed on a stretched polyvinyl alcohol film to prepare apolarizing membrane.

The second transparent protective film (on which the diffusing layer wasprovided in Example 1) was saponified and laminated on one surface ofthe polarizing membrane with polyvinyl alcohol adhesive, so that thesecond transparent protective film (cellulose acetate film) might becontact with the membrane.

The first transparent protective film prepared in Example 1 waslaminated on the other surface of the membrane with polyvinyl alcoholadhesive, so that the slow axis of the film might be parallel to thetransmission axis of the polarizing membrane.

Thus, a viewer-side polarizing plate for VA mode was produced.

(Production of Backlight-Side Polarizing Plate)

Iodine was adsorbed on a stretched polyvinyl alcohol film to prepare apolarizing membrane.

The first transparent protective film produced in Example 1 waslaminated on one surface of the membrane with polyvinyl alcoholadhesive, so that the slow axis of the film might be parallel to thetransmission axis of the polarizing membrane.

A commercially available cellulose triacetate film (Fujitac TD80, FujiPhoto Film Co., Ltd.) was saponified and laminated on the other surfaceof the polarizing membrane.

Thus, a backlight-side polarizing plate was produced.

(Production of Liquid Crystal Display of VA Mode)

A pair of polarizing plates and a pair of optical compensatory sheetswere removed from a commercially available liquid crystal display of VAmode (VL-1530S, Fujitsu, Ltd.). In place of the removed members, theabove-prepared viewer-side and backlight-side polarizing plates werelaminated with an adhesive on the viewer-side and backlight-side of thecell, respectively. The viewer-side polarizing plate was placed so thatthe first transparent protective film might be on the liquid crystalcell side and so that the transmission axis might be in the up-downdirection. The backlight-side polarizing plate was placed so that thetransmission axis might be in the left-right direction. Thus, thepolarizing plates were arranged in cross-Nicol position.

In the thus-assembled composition, the distance between the color filterand the light-diffusing layer was 0.59 mm.

The viewing angle of the prepared liquid crystal display was measured bymeans of a measuring apparatus (EZ-Contrast 160D, ELDIM) when each ofeight tones of black (L1) to white (L8) was displayed.

As a result, the angle range giving a contrast ratio of 10 or more ismore than 80° along the transmission axis and also at the angle of 45°to the transmission axis.

The angle giving bright tones (L7, L8) seen at 200 (right-leftward),namely the range giving preferred tone characters, was 50° at the anglesof 51° and 45° to the transmission axis.

According to the invention, the polarizing plate improves qualities ofimages displayed by liquid crystal displays of OCB and VA mode. Inparticular, it improves (enlarges) the image qualities (particularly,downward viewing angles) of wide-viewing liquid crystal displays. Theviewing angles (particularly, downward viewing angles) are so enlargedthat the image contrast is hardly lowered, that the tone or black-whiteinversion is prevented, and that the hue is hardly changed.

1. A liquid crystal display of OCB mode which comprises a backlightunit, a backlight-side polarizing plate, a liquid crystal cell of OCBmode and a viewer-side polarizing plate in order, wherein theviewer-side polarizing plate comprises an optically anisotropic layerformed from liquid crystal compound, a first transparent protectivefilm, a polarizing membrane, a second transparent protective film and alight-diffusing layer in order, said viewer-side polarizing plate beingso placed that the optically anisotropic layer formed from liquidcrystal compound is arranged on a side of the liquid crystal cell,wherein the first transparent protective film is a cellulose acetatefilm having a Re retardation value of 20 to 70 nm and a Rth retardationvalue of 100 to 500 nm, and wherein the light-diffusing layer comprisestransparent resin and transparent fine particles dispersed therein, saidtransparent resin and said transparent fine particles having refractiveindices that are different from each other.
 2. The liquid crystaldisplay as defined in claim 1, wherein the first transparent protectivefilm is a cellulose acetate film having a thickness of 10 to 70 μm, andcomprising cellulose acetate having an acetic acid content of 59.0 to61.5%.
 3. The liquid crystal display as defined in claim 1, wherein thefirst transparent protective film is a cellulose acetate film comprising100 weight parts of cellulose acetate and 0.01 to 20 weight parts of anaromatic compound having at least two aromatic rings.
 4. The liquidcrystal display as defined in claim 1, wherein the second transparentprotective film is a cellulose acetate film having a thickness of 10 to70 μm, and comprising cellulose acetate having an acetic acid content of59.0 to 61.5%.
 5. The liquid crystal display as defined in claim 1,wherein the second transparent protective film has, on a side of thelight-diffusing layer, a surface on which average surface roughnessmeasured at a cut-off value of 0.8 mm per 100 mm length is 0.2 μm orless.
 6. The liquid crystal display as defined in claim 1, wherein theliquid crystal compound is a discotic liquid crystal compound.
 7. Theliquid crystal display as defined in claim 1, wherein the differencebetween the refractive index of the transparent resin and the refractiveindex of the transparent fine particles is in the range of 0.02 to 0.15.8. The liquid crystal display as defined in claim 1, wherein thetransparent fine particles have a size distribution having at least twopeaks, one of which is in the range of 0.5 to 2.0 μm and another ofwhich is in the range of 2.0 to 5.0 μm.
 9. The liquid crystal display asdefined in claim 1, wherein the light-diffusing layer has a haze of 40%or more.
 10. The liquid crystal display as defined in claim 1, wherein alow-refractive index layer having a refractive index of 1.35 to 1.45 isprovided on the light-diffusing layer.
 11. The liquid crystal display asdefined in claim 1, wherein the liquid crystal cell of OCB mode has acolor filter, and a distance between the color filter and thelight-diffusing layer of the viewer-side polarizing plate is 0.6 mm orless.
 12. The liquid crystal display as defined in claim 1, wherein theliquid crystal cell of OCB mode comprises a backlight-side substrate, aliquid crystal layer and a viewer-side substrate in order, wherein acolor filter is placed between the liquid crystal layer and theviewer-side substrate, and wherein a total thickness of the viewer-sidesubstrate, the optically anisotropic layer of the viewer-side polarizingplate, the first transparent protective film of the viewer-sidepolarizing plate, the polarizing membrane of the viewer-side polarizingplate and the second transparent protective film of the viewer-sidepolarizing plate is 0.6 mm or less.
 13. A liquid crystal display of VAmode which comprises a backlight unit, a backlight-side polarizingplate, a liquid crystal cell of VA mode, and a viewer-side polarizingplate in order, wherein the viewer-side polarizing plate comprises afirst transparent protective film, a polarizing membrane, a secondtransparent protective film and a light-diffusing layer in order, saidviewer-side polarizing plate being so placed that the first transparentprotective film is arranged on a side of the liquid crystal cell,wherein the first transparent protective film is a cellulose acetatefilm having a Re retardation value of 20 to 70 nm and a Rth retardationvalue of 100 to 500 nm, and wherein the light-diffusing layer comprisestransparent resin and transparent fine particles dispersed therein, saidtransparent resin and said transparent fine particles having refractiveindices that are different from each other.
 14. The liquid crystaldisplay as defined in claim 13, wherein the first transparent protectivefilm is a cellulose acetate film having a thickness of 10 to 70 μm, andcomprising cellulose acetate having an acetic acid content of 59.0 to61.5%.
 15. The liquid crystal display as defined in claim 13, whereinthe first transparent protective film is a cellulose acetate filmcomprising 100 weight parts of cellulose acetate and 0.01 to 20 weightparts of an aromatic compound having at least two aromatic rings. 16.The liquid crystal display as defined in claim 13, wherein the secondtransparent protective film is a cellulose acetate film having athickness of 20 to 70 μm, and comprising cellulose acetate having anacetic acid content of 59.0 to 61.5%.
 17. The liquid crystal display asdefined in claim 13, wherein the second transparent protective film has,on a side of the light-diffusing layer, a surface on which averagesurface roughness measured at a cut-off value of 0.8 mm per 100 mmlength is 0.2 μm or less.
 18. The liquid crystal display as defined inclaim 13, wherein the difference of between the refractive index of thetransparent resin and the refractive index of the transparent fineparticles is in the range of 0.02 to 0.15.
 19. The liquid crystaldisplay as defined in claim 13, wherein the transparent fine particleshave a size distribution having at least two peaks, one of which is inthe range of 0.5 to 2.0 μm and another of which is in the range of 2.0to 5.0 μm.
 20. The liquid crystal display as defined in claim 13,wherein the light-diffusing layer has a haze of 40% or more.
 21. Theliquid crystal display as defined in claim 13, wherein a low-refractiveindex layer having a refractive index of 1.35 to 1.45 is provided on thelight-diffusing layer.
 22. The liquid crystal display as defined inclaim 13, wherein the liquid crystal cell of VA mode has a color filter,and a distance between the color filter and the light-diffusing layer ofthe viewer-side polarizing plate is 0.6 mm or less.
 23. The liquidcrystal display as defined in claim 13, wherein the liquid crystal cellof VA mode comprises a backlight-side substrate, a liquid crystal layerand a viewer-side substrate in order, wherein a color filter is placedbetween the liquid crystal layer and the viewer-side substrate, andwherein a total thickness of the viewer-side substrate of the liquidcrystal cell, the first transparent protective film of the viewer-sidepolarizing plate, the polarizing membrane of the viewer-side polarizingplate and the second transparent protective film of the viewer-sidepolarizing plate is 0.6 mm or less.